"The whole sort of determination of whether we were in the interstellar medium or not has been one that's probably taken a good six to eight months to properly evaluate and nobody wanted to jump to any conclusions very quickly," Zank explained.

Zank pointed out that space isn't a cold vacuum containing nothing, as it is sometimes imagined. Besides the comets, asteroids, planets and other objects, it has charged particles just like our atmosphere, only far fewer. "(Space is) much less populated compared to the atmosphere, but because the particles are charged, they carry with them electric and magnetic fields, and the way those particles move around leads actually to the creation and formation of boundaries and structures not unrelated to things you see on the Earth," Zank said.

"For example you'll see shock waves on the Earth that are created by big explosions ... and you see the same kinds of things in the interstellar or interplanetary medium - big shocks, big structures and big boundaries," he said. "It's just that they're bigger and broader because there are fewer particles around, and the processes by which they are formed are a little bit different. But in some conceptual sense, they are similar."

Voyager has two instruments that measure very-energetic particles the spacecraft encounters. About eight months ago, the lower-energy subset of those high-energy particles "just sort of disappeared completely. It was like falling off a cliff," Zank said. "They were there and, suddenly, in a very short space of time they were gone."

By contrast, the most energetic particles - the so-called galactic cosmic rays - "jumped over that same period to a very high level."

Those measurements were "in principle, clear evidence that we'd crossed something, crossed a boundary, and the most likely explanation was that we had crossed into the interstellar medium."

The problem lay in measurements from another instrument - the magnetometer - that Zank described as "an extremely long boom that looks like a ladder." It measures very small magnetic fields and was showing no change in the magnetic field around the spacecraft. "That confused everything," Zank said. Scientists thought there should be changes. Perhaps Voyager hadn't left the Solar System at all, but had crossed into another "complicated structure that we don't understand."

Six months later, still other measurements showed that protons in the low-frequency waves similar to radio waves in Voyager's vicinity were jiggling or oscillating with respect to the background activity. The jiggle "sets up a little electric field" that Voyager could measure. The rate of the jiggle is determined by the number of particles. The more jiggle, the more particles.

It turned out that Voyager was detecting "a very large number of particles compared to what we had been seeing." Put the two findings together - the drop in one kind of particles and the increase in others - and "the conclusion is inescapable," Zank said. "Voyager is in the interstellar medium."